Wearable device for bioelectrical interaction with motion artifact correction means

ABSTRACT

The invention relates to a wearable device arranged for enabling a bioelectrical interaction with an individual when being brought into contact with the individual&#39;s skin I, said device comprising electrodes  2  arranged to carry out said interaction by means of a first electrical signal, said electrodes comprise motion artifact detection means  4.  The electrode assembly  1  comprises the electrode  2  arranged to carry out a bioelectrical interaction with the individual by means of measuring an electrical signal on the body of the individual I and/or applying an electrical signal to the body of the individual. In order to enable said bioelectrical interaction the electrode  2  is provided with an electrical cable  2   a.  According to the invention, the electrode  2  is arranged with a pressure sensor  4.  The pressure sensor  4  is arranged to measure a normal force component of an external force applied to the electrode due to movement of the individual. Preferably the pressure sensor  4  is a force/pressure sensitive resistor or a capacitive pressure sensor. The pressure sensor is arranged to produce an electrical signal, referred to as the second signal, the magnitude of the second signal being proportional to the pressure load. The pressure sensor  4  is arranged with cables  4   a,    4   b  for purposes of signal processing.

The invention relates to a wearable device arranged for enabling abioelectrical interaction with an individual by means of a first signal,said device comprising an electrode having a contact surface arranged tobe brought into contact with the individual's skin for carrying out saidinteraction, the electrode comprising motion artifact detection means.

The invention further relates to an electrode assembly comprising anelectrode having a contact surface, the electrode assembly beingarranged to enable a bioelectrical interaction with an individual bymeans of a first signal when the contact surface is brought in contactwith the individual's skin, said electrode assembly comprising motionartifact detection means.

An embodiment of a wearable device as set forth in the opening paragraphis known from U.S. Pat. No. 5,978,693. The known device is suited toperform a skin-mounted recording of an electrical signal from theindividual by means of an electrode assembly. Such a device is suitableto carry-out a measurement of a signal related to a physiologicalcondition of the individual. The known device is particularly suited tooperate in situations when the individual is not immobilized. The signalmeasured on the body of the individual is further referred to as thefirst signal. It must be noted that the known device is also suitable toperform another type of bioelectrical interaction with the individual,namely an application of an electrical pulse to the individual's skin.Under the definitions of the current application, such an electricalpulse is also referred to as the first electrical signal. In the firstembodiment of the known device the first signal is analyzed byprocessing means of the wearable device in order to monitor thephysiological condition. The monitoring can be carried out based, forexample on the absolute value, a frequency or another suitablecharacteristic of the first signal. In the second embodiment of theknown device (application of electrical signals to the individual) theabsolute value of the applied signal is determined by the processingmeans prior to said application. It is acknowledged in the art thatmotion artifacts occur when the electrodes are displaced due tomovements of the individual, leading to erroneous first signalprocessing. The known electrode assembly comprises motion artifactdetection means arranged to enable the motion artifact correction. Themotion artifact detection means of the known device comprise adeformation sensor. The deformation sensor is positioned on the surfaceof the electrode, covering a part of the area of the electrode. Adeformation in the electrode's geometry, like stretching or bendingyields a signal on the deformation sensor. The processing means of theknown device is arranged to carry out signal processing of the signalfrom the deformation sensor in order to perform the motion artifactcorrection.

A disadvantage of the known device is twofold. First, the motionartifact detection means is complicated in its design, adding to themanufacturing cost of the device as a whole. Secondly, the knowndeformation sensor is arranged to cover only a small area of theelectrode assembly thus resulting in inaccuracy of artifact correction,as the signal from the deformation sensor is not always representativeof the deformation in the geometry of the electrode as a whole.

It is an object of the invention to provide a wearable device where themotion artifact correction is relatively simple with reliable results.

The wearable system according to the invention is characterized in thatthe motion artifact detection means is arranged to determine a componentnormal to the contact surface of an external force applied to theelectrode under operating conditions, said motion artifact detectionmeans being arranged to provide a second signal and comprising motionartifact correction means to process the second signal in order tocorrect the value of the first signal for a motion artifact. Thetechnical measure of the invention is based on the insight that the mostsignificant motion artifacts occur from pushing on the electrodes.Therefore, normal force is a more relevant component than oblique forcecomponents. Additionally, sensors arranged to measure force are cheapand mechanically robust and can easily be attached to the electrodewithout sufficiently increasing the weight of the assembly. The artifactdetection means of the wearable device according to the invention isarranged to measure a signal related to the normal component of theforce, further referred to as the second signal. The second signal ismade available to the motion artifact correction means for motioncorrection. After the motion correction has been carried-out the firstelectrical signal can be processed further leading to a more accurateinteraction with the individual. In the context of the present inventionthe interaction with the individual comprises measuring an electricalsignal on the body of the individual and/or applying an electricalsignal to the body of the individual. A preferred embodiment of themeasuring of an electrical signal on the body of the individualcomprises a measurement of a cardiac activity of the individual. Apreferred embodiment of the application of an electrical signal to thebody of the individual comprises a myostimulation or an application ofan electrical signal for other purposes.

An embodiment of the device according to the invention is characterizedin that that the device comprises control means arranged to analyze thesecond signal, said control means being further arranged to actuate themotion artifact correction means upon an occurrence of a predeterminedevent. The wearable device according to the invention is conceived to bedurably worn by the user. It is therefore preferable that the powerconsumption of the electronics of the wearable device is minimized.Preferably, the device according to the invention is adapted to carryout motion artifact correction only upon request. Therefore the deviceaccording to the invention is preferably arranged with control meanswhich actuate the motion artifact correction means upon a predeterminedevent. An example of such a predetermined event is an actuation of amotion artifact correction button supplied on the exterior of thedevice. Alternatively, the motion artifact correction means can beactuated automatically by the control means in case the second signalexceeds an allowable threshold level. It is possible that more than onethreshold level is assigned. For example, for each activity of the user,like sitting, walking, jogging, etc a corresponding threshold can beassigned. An actual threshold level corresponding to the actual activityof the individual can be downloaded for example from a look-up tablestored in a memory unit of the device. It is possible that the type ofthe actual activity of the individual is set by a user interface, or isdeduced from the first signal by proper signal processing. The userinterface can be arranged with actuation buttons and/or voicerecognition.

A further embodiment of the device according to the invention ischaracterized in that that the device comprises means for deriving theoccurrence of the predetermined event from the second signal. The loadresulting from the normal component of the external force applied to theelectrodes is best assessed by a pressure sensor. The result of theforce acting on the electrode is a change of the electrical potential atthe boundary electrode-skin. Forces on the electrode primarily altercapacities between the electrode and the skin. A preferred embodiment ofa pressure sensor is a force sensitive resistor. Due to this embodimentof the wearable device a high correlation between the force acting onthe electrode and the skin potential produced by that force isdetermined.

A still further embodiment of the wearable device according to theinvention is characterized in that the pressure sensor comprises a thinfilm. In order to measure the forces acting on the electrodes a thinfilm can be positioned on the rear surface of the electrode. Preferably,the pressure sensor comprises a thin film force/pressure sensitiveresistor manufactured according to a thin film technology or acapacitive pressure sensor manufactured according to the thin filmtechnology. The principle of operation of the said pressure sensitivesensors is known per se in the art and will not be explained in detailhere. In the device according to the invention the output signal fromthe pressure sensor is made available to the motion artifact correctionmeans of the device in order to perform the motion artifact reduction inthe first signal by means of a suitable signal processing of the outputsignal from the pressure sensor. In order to carry out the motionartifact reduction accurately a calibration of relation between theoutput signal of the sensor and the absolute value of the motionartifact is required. In a preferred embodiment the signal processingcomprises a correction for non-linearity in the relation between theabsolute value of the motion artifact and the output signal from thepressure sensor. Said relation is linearized by means of a suitablesignal processing. An example of such a suitable signal processing is alinearization step carried out by means of a look-up table. In case theoperation of the device is controlled by a microprocessor, theestimation of a required signal linearization can be downloaded from acalibrated look-up table stored in the microprocessor of the device. Thelook-up table comprises non-linearity correction data for a non-linearrelation between the actual value of the external force applied to theelectrodes and the output signals from the pressure sensor. After theoutput signal from the pressure sensor is linearized the absolute valueof the signal is then supplied to the motion artifact correction means.Preferably, the motion artifact correction means comprises an adaptivefilter. The operation of the device will be illustrated using an examplewhere the device according to the invention is arranged to carry out aperson monitoring task. The first signal thus corresponds to a signalmeasured by the electrodes on the body of the individual, for example anECG signal. In case the individual being monitored is moving, theabsolute value of the first signal is distorted due to the motionartifact. The pressure sensor arranged on the rear surface of theelectrodes provides the second signal which is a measure of the motionartifact. Preferably, the electrodes are of a dry type requiring no gelto be applied between the individual's skin and the measuring surface ofthe electrode. Suitable materials to manufacture a dry-type electrodeare conducting rubber, plastic or textile. However, the output signal ofthe pressure sensor is not always linear with respect to the externalforce applied to the electrodes due to the motion. Therefore in anembodiment of the device according to the invention a signallinearization step is foreseen. Preferably, this linearization step iscarried out by means of the look-up table. After the output signal fromthe pressure sensor is corrected for non-linearities, the absolute valueof the output signal is supplied to the adaptive filter, where adaptivenoise cancellation is performed. The principle of the adaptive noisecancellation is known per se in the art. Preferably a subtraction of thesecond signal from the first signal is carried out. Preferably, thesubtraction operation is carried out digitally.

An electrode arrangement according to the invention is characterized inthat the motion artifact detection means is arranged to determine acomponent normal to the contact surface of an external force applied tothe electrode under operating conditions, said motion artifact detectionmeans being arranged to provide a second signal, related to the externalforce.

These and other aspects of the invention will be explained withreference to Figures.

FIG. 1 shows schematically an embodiment of the electrode assemblyaccording to the invention.

FIG. 2 shows schematically an embodiment of the device according to theinvention comprising control means.

FIG. 3 shows schematically an embodiment of the motion artifactcorrection means according to the invention.

FIG. 4 shows schematically an embodiment of the wearable deviceaccording to the invention.

FIG. 1 shows schematically an embodiment of the electrode assemblyaccording to the invention. The left-hand part of the Fig. shows anassembled electrode, whereas the right-hand part shows the componentsfrom which the electrode is assembled. The electrode assembly Icomprises the electrode 2 arranged to carry out a bioelectricalinteraction with the individual by means of measuring an electricalsignal on the body of the individual I and/or applying an electricalsignal to the body of the individual. In order to enable saidbioelectrical interaction the electrode 2 is provided with an electricalcable 2 a. For purposes of measuring a signal from the body of theindividual, the cable 2 a is connected to an input of a preamplifierfollowed by a suitable signal processing circuit (not shown). Forpurposes of an application of an electrical signal to the body of theindividual, the cable 2 a is connected to the output of a power supplysource (not shown). The corresponding electrical circuits are known perse and fall within the scope of knowledge of the person skilled in theart. According to the invention, the electrode 2 is arranged with apressure sensor 4. The pressure sensor 4 is arranged to measure a normalforce component of an external force applied to the electrode due tomovement of the individual. Preferably the pressure sensor 4 is aforce/pressure sensitive resistor or a capacitive pressure sensor. Thepressure sensor is arranged to produce an electrical signal, referred toas the second signal, the magnitude of the second signal beingproportional to the pressure load. The pressure sensor 4 is arrangedwith cables 4 a, 4 b for purposes of signal processing.

FIG. 2 shows schematically an embodiment of the device according to theinvention comprising control means. The electrode assembly I is providedwith three outputs, 2 a, 4 a and 4 b. The output 2 a is a signal outputfrom the electrode yielding the first signal S to be processed by theelectronics (not shown) of the device according to the invention. Thesignal from the output 4 a, 4 b of the pressure sensor, definedpreviously as the second signal, is supplied to the input of anamplified 8. The amplified signal M, characteristic to the motionartifact, is then processed by a high pass filter 10, a full waverectifier 12 and a low pass filter 14. The resulting signal is thensupplied to the input of control means 16, where the second signal isfurther processed. According to the invention the control means 16 isarranged to actuate the motion artifact correction means 20 upon anoccurrence of a predetermined event. Examples of the predetermined eventare an actuation of a motion-correction button on a user interface (notshown), an actuation of a voice recognition routine on a user interfacewhere a demand to switch on the motion artifact correction isrecognized. In a preferred embodiment the event corresponds to thesecond signal exceeding a predetermined threshold level. Preferably thecorresponding threshold is stored in a memory unit 18. Upon anoccurrence of the event the motion artifact correction means 20 areactuated and the motion artifact correction is carried out. For thispurpose the first signal S and the processed second signal M aresupplied to the input of the motion artifact correction means 20.

FIG. 3 shows schematically an embodiment of the motion artifactcorrection means according to the invention for a case where a pluralityof electrode assemblies 1 a, 1 b are arranged on the body of theindividual (not shown). The second signals M1, M2 related to the motionartifact of the electrodes 1 a and 1 b are amplified by means of theinput amplifiers 8 a, 8 b. A the next step the signals M1, M2 aresubtracted from each other by a first subtracting means 21 to obtain anaverage value of the signal to rule out electrode's differences. Theresulting averaged second signal M is processed together with the firstsignal S by the motion artifact correction means 25, comprising anadaptive filter 22 and a second subtracting means 24. The resultingsignal is the corrected first signal which is supplied to the processingmeans 26 of the device according to the invention. Preferably the motionartifact correction means comprises a linearization element (not shown)carrying out a linearization of the second signals M1 and M2. Thelinearized second signal is then supplied to the linear input of theadaptive filter 22.

FIG. 4 shows an embodiment of a technical realization of a wearabledevice according to the invention. The wearable device 30 comprisesmonitoring means 31 arranged to monitor a physiological condition of theuser. The monitoring means 31 comprise a set of electrodes 31 a, 31 b tobe arranged on the body of the user to pick-up a signal characteristicof the targeted physiological condition, for example an ECG signal. Theelectrodes 31 a and 31 b comprise artifact detection means [not shown]in order to provide a signal characteristic to a motion experienced bythe electrodes under the operating conditions. Additionally, themonitoring means 31 can comprise a sensor 32 arranged to monitor asignal not directly related with the targeted physiological condition.An example of such a sensor is a blood pressure sensor or a respirationrate sensor. The monitoring means 31 are arranged to perform acontinuous monitoring of the physiological condition of the user and arefurther arranged to provide a corresponding signal to the front-endelectronics 40 of the system 30. The monitoring means 31 and thefront-end electronics 40 are worn on the body of the user, preferably atthe waist area. Examples of suitable carriers for the wearable deviceare known per se in the art. The front-end electronics 40 is arranged toanalyze the signal from the electrodes 31 a, 31 b. In case the signalcomprises no motion artifact it is processed as such, otherwise thefront end electronics 40 performs the motion artifact correction, forexample according to the embodiments shown in FIG. 2 and FIG. 3. Forthat purpose the front-end electronics 40 comprises a preamplifier 41and analogue processing circuit 42, an ADC unit 43, detection means 45and a μ-processor 44. Optionally the front-end electronics 40 comprisesalarm means 46 and transmission means 47. The detection means 45comprises a sensor signal interpretation unit 45 a and featureextraction means 45 b. The system 30 operates as follows: the monitoringmeans 31 acquires the raw data which are delivered to the front-endelectronics 40. The front-end electronics 40 provides means forreceiving the signals from the monitoring means, performs suitedanalogue processing by means of the analogue processing circuit 42. Herethe signal from the electrodes is tested on motion artifact. In case thesignal from the pressure sensor indicates that the artifact isacceptable, the signal from the electrodes is processed as such.Otherwise, the signal from the pressure sensor is used to correct therunning value of the signal from the electrodes. The processed raw datais converted into a digital format by means of the ADC 43 and isforwarded by a μ-processor 44 to the detection means 45, where thecondition of the user is being analyzed. For cardiac applications thedetection means 45 comprise a per-se known QRS-detector to determine R-Rpeak intervals in heart cycles. The detection means 45 comprise a sensorsignal interpretation unit 45 a arranged to derive a feature in thesignal characteristic of an abnormal physiological condition of theuser. For cardiac applications, for example said feature can be afrequency of the signal. In case the detection means 45 detects theabnormal condition, a signal is sent to the alarm means 46 to generatean alarm, which is transmitted by the transmitting means 47, for exampleby means of a RF-link to warn a bystander or specialized medicalpersonnel.

1. A wearable device (30) arranged for enabling a bioelectricalinteraction (M,S) with an individual by means of a first signal (S),said device comprising an electrode (2) having a contact surfacearranged to be brought into contact with the individual's skin (I) forcarrying out said interaction, the electrode comprising motion artifactdetection means (4), characterized in that the motion artifact detectionmeans (4) is arranged to determine a component normal to the contactsurface of an external force applied to the electrode under operatingconditions, said motion artifact detection means being arranged toprovide a second signal (M) and comprising motion artifact correctionmeans (20) to process the second signal in order to correct the value ofthe first signal for a motion artifact.
 2. A wearable device accordingto claim 1, characterized in that the device comprises control means(16) arranged to analyze the second signal (M), said control means (16)being further arranged to actuate the motion artifact correction means(20) upon an occurrence of a predetermined event.
 3. A wearable deviceaccording to claim 2, characterized in that the device comprises meansfor deriving the occurrence of the predetermined event from the secondsignal.
 4. A wearable device according to claim 1, characterized in thatthe motion artifact detection means (1 a) comprises a pressure sensor(4) arranged on a rear surface of the electrode (2), said rear surfacebeing opposite the contact surface.
 5. A wearable device according toclaim 4, characterized in that the pressure sensor (4) comprises a thinfilm.
 6. A wearable device according to claim 1, characterized in thatthe motion artifact correction means comprises a linearization element,said linearization element being arranged to correct for a non-linearityin a relation between an absolute value of the second signal and theexternal force.
 7. A wearable device according to claim 1, characterizedin that the electrodes are of a dry type.
 8. An electrode assembly (1)comprising an electrode having a contact surface, the assembly beingarranged to enable a bioelectrical interaction with an individual bymeans of a first signal (S) when the contact surface is brought incontact with the individual's skin, said electrode assembly comprisingmotion artifact detection means (4), characterized in that the motionartifact detection means (4) is arranged to determine a component normalto the contact surface of an external force applied to the electrodeunder operating conditions, said motion artifact detection means beingarranged to provide a second signal (M), related to the external force.9. An electrode assembly according to claim 8, characterized in that theartifact detection means (4) comprises a pressure sensor arranged on arear surface of the electrode, said rear surface being opposite thecontact surface.
 10. An electrode assembly according to claim 9,characterized in that the pressure sensor comprises a thin film.